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What effect would there be if the brakes were locked at touchdown?
I'm imagining if you had a panicky passenger sitting in the right seat who had both feet firmly on the brakes while landing. Unless you had a crosswind and needed rudder as you touched down it's possible you wouldn't be aware of it. Or if there was a malfunction causing them to be locked. How likely is it that could cause you to have a bad day?
What if only one side was locked up (which seems worse to me)?
I'm mainly asking about small aircraft. Would it be any different for an airliner?
What if you were aware of it? What action would you take?

@ymb1 That's one of the more annoying warning alarms I've ever heard

It's mostly harmless in a nose wheel configuration. You quickly blow all tires and slide along the runway on your wheel hubs until the aircraft comes to a rest. Crosswind will make the experience worse, because at lower speed you have less rudder authority and the aircraft might leave the runway.
If only one side is locked you will risk a ground loop. Here it is best to step on the other brake so both cause the same braking force.
But this is peanuts when compared to a tailwheel configuration. Here the consequence is a headstand right after you put some load on the wheels, which can be right at touchdown when you come down with some sink speed. Depending on the speed of the aircraft when this happens and the length of the fuselage nose, the aircraft might even flip on its back. Now this is sure to ruin your day. I've witnessed this once with a Polish Yak-12 which landed on soft ground - nothing more. The pilot climbed out unhurt from his inverted position, but damage to the aircraft was extensive. This was the occasion when I learned the worst Polish cursing ever.
If only one side is locked in case of a taildragger, I am unsure what will happen first: Ground loop or headstand. Details depend on the wheel track and the attitude at touchdown. Be sure to touchdown at the lowest possible speed and with the highest nose-up incidence possible to increase the resistance of the aircraft against flippig over. Unload the locked wheel with ailerons, so you touch down on the working wheel only, but make sure that you do this with as little sideslip angle as possible. Flying a gentle turn while touching down will help - just place the touchdown point at the moment when the aircraft is aligned with the runway (I know - easier said than done).
Touching down on one main wheel of a taildragger with some lateral speed by sideslipping into the direction of the working wheel will quickly roll the aircraft into the direction of the lateral speed, which helps to unload the locked wheel but might end up in a three-point landing: One wheel, wingtip and fuselage nose. Again, this is sure to ruin your day.

That is what is colloquially called a "headstand" (Picture source)

For a really big headstand, see: http://www.neopress.in/airlander-10-worlds-largest-aircraft-crashes-in-bedfordshire/

I've seen it happen many times (ask any rental place about this). You get a big bald spot cut into the tread. Sometimes it goes all the way through the tire.
If it happened on one side, you'd likely get a ground loop.

So, costly, but not necessarily catastrophic

Depends on the skill of the pilot, if you are touching down firmly in a cross wind with the upwind tire first, it could be very catastrophic. Usually you can tell if the passenger has their feet on the pedals, which would mean go around and a lecture from me. My preflight safety briefing includes telling the passenger to stay off the pedals.

When Air Transat Flight 326 glided into the Azores after fuel exhaustion over the Atlantic, the pilots were unsure whether they would be able to apply the brakes on their Airbus 330 more than once, and their anti-skid systems were inoperative.  So they applied the brakes and held them, locking the wheels.  The results, from the accident report:

The tires quickly abraded and deflated at a point between about 300 and 450 feet beyond the second and final touch down. The segments of the main wheels contacting the pavement were worn down to the bearing journals, the left, rear, inboard wheel detached from the axle. Both left and right brake anti-torque links attachment horns on the bottom segments of the main oleos also contacted the pavement; the horns were abraded to the point that some of the links separated from the oleo resulting in the rotation of at least one brake carrier. Shedding of brake and wheel components during the landing run also resulted in a combination of punctures and impact damage to the airframe and left engine nacelle.
...
CFR [Crash Fire Rescue] reported that there was fire visible coming from the
main wheels, and responded to put out the fire. At 06:46, CFR reported that the evacuation was in progress and that some signs of fire still existed in the left main wheels. By 06:54, all fires were extinguished; the areas around the main wheels continued to be monitored due to the fire hazard associated with the damaged wheels and hot brakes.

However, the airline stayed on the runway, and everyone survived.  (Two passengers were hospitalized due to their injuries.)

If that happens to an airliner(full brakes at touchdown), a lot can happen.
1- depending on the landing speed, the wheel struts could break off
2- if the struts resist such enormous load and bending force, the n the wheels will wear and burst due to friction and then most certainly burn depending on the condition of the runway(wet or dry).

A locked wheel causes less horizontal load than one with full brakes which is still rotating, thanks to an anti-skid system. The legs will only break off in a Hollywood movie or when the aircraft hits an obstacle at high speed.

@PeterKämpf that's true, locked wheels don't brake as well as rolling ones.


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Does being out of trim affect drag?
If the aircraft is not trimmed correctly and either the pilot or autopilot is having to use constant pressure on the elevator does this increase drag?

This should have the same effect, minus feedback loop issues if the pilot doesn't input exactly the right amount of pressure

I've been told that an aircraft will find its trim speed/attitude if left untrimmed, which leads me to believe that you may be increasing drag by trimming it (aka forcing it to hold an attitude), but somebody a little smarter than me will have to confirm or refute that claim...

@RonBeyer You're forcing it to hold an attitude trimmed or not. My non-pilot intuition tells me it depends on the trim system. If the whole stabilizer moves it prob helps to trim but if it uses trim tabs then it's really just using the elevator anyway. It just helps you hold it in position

@TomMcW Not really, think of it this way, if you are going too slow, the nose will drop and you'll pick up speed, as you pick up speed lift becomes greater and the nose will rise. Same if you are going too fast, your nose will come up, speed will bleed off, lift decreases and the nose drops. Aside from the oscillations after some period of time the aircraft will find its trim speed (even if the trim is neutral).

@RonBeyer Here's my thinking: You're in an ac with a trimmable stabilizer trimmed for climb. When you level off you push forward the yoke forcing the elevator down. You set the a.p. to maintain speed and altitude without re-trimming. The tailplane exerts the appropriate downforce to maintain the wings at the correct AoA for the set speed. In climb config the stab is trimmed slightly downward and now the elevator is also pointing downward to maintain level flight. Seems like it would produce more drag like that than if you re-trimmed so that the elev is neutral and aligned with the stab.

Normally not; details depend on the control system and by how much the aircraft is out of trim. The force produced by the empennage will be the same in the trimmed state and when out of trim, only the control forces are different.
Let's examine the different possibilities:

Adjustable spring (as used on some light aircraft and gliders): The elevator deflection will be identical to the trim position, so there is no difference at all.
Trim or servo tab on the elevator: The tab deflection will be different, but the elevator position will be practically identical. For tab angles of less than 10° relative to the elevator there should not be any difference in drag.
Adjustable stabilizer: Now the elevator deflection will be different, and depending on the trimmed state the deflection angle might cause higher airfoil drag. If the elevator deflection relative to the stabilizer is more than 10°, a slight drag increase can be expected.

If the power consumption of the autopilot is higher in the out of trim case, this might cause higher generator loads and more engine power is needed to keep it powered. But this effect should be very small.

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Is navigation around thunderheads done at the pilot's discretion?
It's my understanding that if a pilot is flying over an ocean or otherwise uncontrolled airspace and they encounter a cumulonimbus, they navigate around it at their discretion.
How does this happen in controlled airspace though? What instructions do ATC and a PIC typically exchange if a PIC wants to request navigation around a thunderhead?

Keep in mind that "over an ocean" is usually still controlled airspace, even if there's no radar coverage. Most flights aren't crossing the North Atlantic at 5,000 ft in Class G airspace. There will be procedures in place (here are the ones for the North Atlantic) for deviations for weather, but absent emergency situations, it's all still controlled.

If you are in controlled airspace its very possible ATC will direct you around weather, you can find out more about that in this question.
However what ATC can see on their scope varies from place to place and weather moves quickly. As such pilots can, and often do request deviation for weather. Lets say you are flying along dodging some clouds here and there, popping in and out, and all of a sudden you pop out with a cell forming in front of you. You may request something like:

N123AB requesting left turn 20 degrees for weather
N123AB left 20 approved for weather avoidance

It may also not be a bad idea to file a PIREP to help others out.
Ultimately as pilot in command you can deviate from any rule to ensure a safe flight. If you were about to fly straight through a storm and ATC was not allowing a deviation or not issuing one a case could be made for deviating any way. You can read more about that here.

Phraseology-wise, the use of the word "Cleared" in your example is wrong. It would either be something like "Left turn [20 degrees] approved, [report turning inbound XXX again]" or "Turn left 20 degrees, vectors, [report clear of weather]"

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What kind of failover system do Airbuses with side sticks have?
Airbuses use joystick-style side sticks for controlling the aircraft. Among the features of this system is that there exists no mechanical connection between the joysticks and the control surfaces.
In case of an electrical failure, what backup system is in place to make sure the pilot still has control of the aircraft?

I read somewhere there are mechanical links for elevator trim and vertical stabilizer allowing minimum control the time the ram air turbine is deployed. I don't know if those control still exist on more recent fly-by-wire aircraft, but if not it means the fly-by-wire system is more reliable than mechanical controls.

In case of a total electrical failure, the aircraft can still be controlled using rudder pedals and manual pitch trim.
The airbus EFCS (Electronic flight control systems) are configured as below:

Airbus EFCS; Image from smartcockpit.com
From Airbus Flight Control Laws:

MECHANICAL BACKUP
In case of a complete loss of electrical flight control signals, the aircraft can be temporarily controlled by mechanical mode.

Pitch control is achieved through the horizontal stabilizer by using the manual trim wheel.
Lateral control is accomplished using the rudder pedals.
Both controls require hydraulic power.

The A340 also seems to have a similar system. From the A340 Flight Deck and Systems Briefing for Pilots:

Mechanical back-up
• To sustain the aircraft during a temporary complete loss of electrical power.
• Longitudinal control of  the aircraft  through trim  wheel. Elevators kept at  zero deflection.
• Lateral control from pedals. Roll damping is provided by the Back up Yaw Damper Unit (BYDU).

A340 mechanical Backup system; image from A340 Flight Deck and Systems Briefing for Pilots
Note that this is given only as a temporary measure. Also, See How are fly-by-wire airliners controlled in case of complete electrical failure? and How does the Airbus flight computer's voting system work?

Do you want to add anything about using differential thrust as well?  There was an accident (United Flight 232) some years ago due to a hydraulic failure where the pilots used differential thrust to safely (if roughly) land in a field.  The fact that anyone survived (over half of the people on board survived) is a testament to getting the most out of the bad situation, per the NTSB.

I wonder if a full electrical failure is in any FBW training curriculum... Rudder pedals and trim wheel control sounds like "make the lawyers happy" but I'm not so sure if its successful real-world outcome.

Well in the event of a total electrical failure, you would - I think - lose engine control as most, if not all, modern airliners use FADEC engine controllers with no mechanical throttle control.  That's why they have multiple electrical buses which can be cross fed and powered by an alternator on each engine, the APU, and finally the emergency RAT.  A failure of all of those systems is less probable than damage to mechanical or hydraulic controls rendering them non functional.

@CarloFelicione -- the FADECs have dedicated engine driven PMGs for backup power

@RonBeyer -- it'd be quite annoying to fly, but the Airbus test pilots did it.

@UnrecognizedFallingObject Test pilots yes, but what I was saying is your everyday Airbus pilot. Technically you can control a Cessna 172 the same way, trim wheel for pitch and rudder for roll/yaw if one of your control wires snapped, but nobody trains for that.

@RonBeyer -- that'd be a very good mainsite question -- "how far do Airbus pilots train for mechanical backup control?"

@UnrecognizedFallingObject If you feel you can make a good question out of that, go for it, don't think I can put that together myself in the next few days.

@RonBeyer -- asked :)

@RonBeyer, I did, and it's even fun, as long as there is no real emergency :) But more importantly, Cessna manual has specific instructions for this situation, so technically it's a 'design case'. (For Cessna, you normally trim for a slow speed and then control pitch/altitude with thrust; only touchdown requires re-trim).

@Zeus Its not just a "design case" its a regulatory requirement. I'll have to dig for the CFR but it basically says that there has to be at least one method of control after the primary method fails.

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What is the exact blue-green color that used to be popular in cockpits of Russian fighters
Does anybody know the exact name of the color that was used to paint all Cold War era Soviet fighter cockpits? It’s a blue green color but I’d like to get the exact color as I’m currently building a model of a Soviet fighter. I need to paint the cockpit.
The image below is of a MiG-23 cockpit for reference.

How a color is defined or represented is an extremely deep rabbit hole and there is no simple answer. Even the Soviets/Russians probably mixed some blue and green and said "good enough". If you're only interested in painting a model, maybe this product (which I found by googling, no affiliation) is suitable for you?

To my eyes, on my monitors, this looks close to Pantone 325. YMMV. Even with that I didn't see a close match in the Humbrol Shade Chart, which also lists Tamiya and other popular brand equivalents.

I have seen the color described as "Russian Turquoise Green." It reminds me of that very special shade of baby poop green.

According to this model color chart, Mig uses a turquoise shade for cockpit interiors:

CMYK (54%, 0%, 10%, 43%), which corresponds roughly to the hue of Pantone 10297 coated:

However we don't know how this color chart was designed, nor whether the color was sampled from an actual aircraft or a photo.

Color is a complex matter, specially when trying to compare an image from a file with a reference:

Unless the image is captured and processed with a chain of tools working according to accurate embedded ICC color profiles, the resulting colors are actually variable. To start with most online images don't embed the color profile used for the final step, thus the same color is not guaranteed to be the same in the final file.

Consumer displays aren't really faithful, comparing colors on a screen with colors on a physical support results in differences, often important ones.

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How to deal with larger and larger engines
What are possible solutions, if aircraft manufacturers can't fit ever larger turbofans onto the standard low-wing passenger jet configuration?
Below, we see a 737 with large turbofans. If a new engine generation were to emerge with significantly larger fan diameter, would aircraft manufacturers

change to a high-wing design, as with the C-17
change to a low, gull wing design to make additional room
or apply other solutions

All of these are possible. The question is how much is the effort required to implement those. You might have to develop a new airframe. This in turn will have different flight characteristics, which means extensive test schedule. That will then cause the need for pilots to qualify for that airplane. All of which add to the costs of buying and operating the plane. Thus the reason for Boeing trying to avoid it - which kinda came around and put them on the ground the hard way now.

@ymb1 I am aware, that there can't be one answer to this. However, there could be arguments that make one approach preferable over others. A high wing configuration has been done before. On the other hand, Boing introduced folding wing tips with the 777X, which is something rather new in the world of passenger jets. I am interested in the merits of the approaches I stated, and in possible alternatives.

Engines are not designed in a vacuum.  The engine manufacturers don't go off and design an engine and then show up to Boeing and say, "here's our new engine, it's 98" in diameter, now try to fit it on your plane".  The engine and the airframe are designed together, in parallel, as a partnership.  Fan diameter will be agreed upon by the two companies as they both try to design the optimal system. If the airframer decides the benefit of a bigger fan outweighs the cost of a wing redesign, then they will do it. Otherwise the fan will stay the same size as previous generation.

The easiest way to accommodate a larger engine under the wing is to increase the landing gear size. This has some other implications like more space needed in the landing gear bay (although there are clever solutions like this one for the 737 MAX 10) and higher weight, but if the larger engines with better efficiency outweigh the costs for these changes, it will certainly be done.
Changing the design to high-wing or tail mounted engines comes with other drawbacks, as already discussed here:

How does the mounting location of a jet engine affect aircraft performance?
What are the pros and cons of high-wing compared to low-wing design?

I think tail mounted engines, where there is more ground clearance would best solve the problem.
See Picture A and B
Picture A

Picture B

Over the years one of the main problems with tail mounted engines has been difficult maintanance, however with larger engines mounted at the tail, leaving as much clearance as the smaller wing mounted engines of today, that ceases to be a problem.  Secondly adjustable scarfolding hydraulic powered will elevate the mechanic, technician or engineer and his tools closer to the engine.
Source picture A
Source Picture B

While empennage mounted, hi-bypass turbo fans may come in the future, there are quite a few drawbacks to mounting them there, which is why there are so few transport category aircraft being built with them today. This question explains a lot.


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Can you land IFR at sea?
I've heard that some seaplane bases have IFR approaches, but what about landing at sea?
If you flew out into international airspace, could you land at sea and then taxi back into US waters without being subject to US regulations? You wouldn't have to know where the "runway" is because it's everywhere.
(FYI, I'm a SEL pilot, so I'm not considering actually trying this!)

I have no idea. Is an airplane taxiing at sea treated as a "ship" for regulatory purposes? What I want to know is if you can land in international airspace in conditions that you wouldn't be able to in US airspace. There are no valid IFR approaches to the middle of the ocean.

IFR flights can perform visual approaches

visual approaches to a runway on a controlled aerodrome, yes, but nowhere else

@ymb1 if you mean airspace as in FIR, yes. But a FIR's borders are not the same as nation borders

I think you need to clarify some things in your question.  First, I think you mean "IMC", and not IFR.  Second, there is no such thing as an "IFR approach" (as a physical thing -- thus, one cannot "be installed" at an airport).
If you're referring to the weather and/or visibility conditions, then you mean IMC.  If you are flying under a certain set of rules (IFR), it does not matter whether you can see things or not

What do you mean by "at sea"?

@JonathanWalters By "at sea" I mean international water, under international airspace, beyond the ADIZ airspace. Is it legal? Is it safe?

@Jimmy When I said "be installed", I was thinking of the physical equipment that is associated with an IFR approach, such as ILS systems, VOR stations, or NDBs. Obviously, none of these would ever be found at sea, not that it would matter since you don't have to hit a 200'x3000' strip of runway-- any spot on the water is as good as any other spot as long as you don't hit an island, ship, or other rare obstacle. You should be able to land anywhere with a 100' minimum DH.

@ThrowawayAccount3Million Seaplanes and wing-in-ground craft are addressed by the COLREGs. When operating on the water, they have the lowest possible precedence (below all other watercraft).

OK. There are some answers with some good information here but here is my stab at it. I'm not going to go into whether this is smart or not, just possible. And yes you can land a seaplane safely in zero-zero conditions, if the conditions are right (haha). Anyone that tells you different isn't using their imagination and/or isn't a seaplane pilot. Once again, not saying it's smart, just possible.
1) "I've heard that some seaplane bases have IFR approaches, but what about landing at sea?"
If you have an ILS, you need to mount the antenna somewhere and "at sea" is not very conducive to that. This leaves GPS (RNAV) approaches that could be theoretically built "at sea".  However, until RNP and LPV (or however they are renaming these concepts this week) becomes common place in small aircraft you can only get down to minimums of about 500' anyways. If you are dropped off that "high" , you are pretty much legal "at sea" to be down there buzzing around VFR anyways even in US airspace (assuming you can see,  which if you couldn't then landing from an IFR approach wouldn't be prudent anyways, even on a runway). So it kind of defeats the point of doing an approach in the first place.
2) "If you flew out into international airspace, could you land at sea and then taxi back into US waters without being subject to US regulations?"
Short answer, technically yes, but….. You could land outside of ADIZ, which is typically 12 miles out and be subject to no FARs, I guess, technically. However there is almost always a controlling agency for that airspace even if the country possibly care 'as much' once outside their sovereign airspace. They will start caring a lot if and when you conflict with their traffic. Back to the point, there are many practical problems with landing this far out  though and it certainly isn't worth the hassle just to avoid some regulations (that don't really limit you). First problem, there are very few seaplanes still flying today that can land 12 miles out to sea on anything but the absolute best (read that as calmest) day.  And, oh yes I forgot to mention, in the US at least they aren't going to wait until you get right up to twelve miles with no transponder code to intercept you (I'm assuming you aren't talking to anyone because you don't want to be subject to US regs).  So you actually need to land about 35 miles out (don't ask me where that number is from, I honestly don't remember, but 35 miles  was the magic number and deadline for having a transponder code to cross the ADIZ). Moving on,  If you do find that rare day when the water is just beautifully calm and do land, you run into problem number 2. Second problem, now you have to taxi 35 miles to shore. Talk to any seaplane pilot you can find, and they will be happy to tell you how painful this would be. Even if it were just twelve miles this would be a great reason to just talk to someone and go the flying route. This leads us to problem 3, which is even if you aren't subject to aviation regulations you are now a boat. So you must clear immigration and customs as a boat. The coast guard whom you may run into will not be a problem. In my experience they are generally great people and very practical because they have important jobs to do. They'll call a hammer a hammer, and will be happy to treat you like a boat. Your problem will be with customs and border patrol. They will be so confused by this beast you have brought them, that they MAY not want to clear you just for fear of getting reprimanded. Be ready for lengthy explanations and calls to supervisors leading to most likely hours of waiting. Not saying CBP doesn't do an excellent job, but you just brought some poor junior officer a seaplane which is the equivalent to taking a puffer fish to a butcher in Alabama and telling him, "Don't worry, I'm pretty sure my wife won't sue you if you kill me by cutting that thing wrong." From personal experience, if you try to set something like this up in advance with CBP you have about a five percent chance of it actually slightly helping somewhat, but not really. Problem is when you call to set something up you are talking with supervisors and they are definitely not the ones who are going to meet you, and they probably can't tell you who will meet you.….
All that being said, I don't see any reason to avoid US airspace. Im not sure what regs you think you'd be avoiding but there aren't really that many to worry about in this situation (I realize this is probably why you asked in the first place). There are no regulations against landing in salt water anywhere in the US. The coast guard owns the salt water essentially and they are great to work with. Also controllers aren't going to care if you land without an airport anywhere, no regs against it (think helicopters, grass strips, good bush flying). That being said, there is a big difference between being legal and not pissing people off. Land in the intercoastal in a lot of places and you'll probably piss someone off.  Land elsewhere in the intercoastal and not so much. Pissing people off can be much worse at times than legal issues.
For the record I'm just an old seaplane bum with a number of years dealing with regulatory agencies, so take my answer for what it's worth (probably not much).

The reason to avoid US airspace is to commute to work in SF by air. The only nearby airport, SFO, has a $200 landing fee, so getting there by ASEL is out of the question, flight plan or not. Also, there are no public-use seaplane bases, so you can only land in the bay in good VFR weather (assuming there isn't a local law against it). That leaves only the open sea, which you still need VFR to legally land in, except maybe if you land far enough out, which you just explained what the ramifications are. So that leaves only driving in SF traffic. :( But I don't have my SES ticket anyway.

Don't need a seaplane base to land. No regs against it in SF (salt water, as I said). I knew the old owner of San Fransisco Seaplanes which looks like it has now become Seaplane Adventures. They operate out of Commodore Center Seaplane Base in Sausalito (which is an old alternate landing strip for the clippers) and the company has been operating in and out of Pier 39 for years. They would know who to talk to in order to get permission to tie up to a dock. I have landed numerous places in San Fransisco bay, the only problem downtown is finding a good spot tie up.

You do need a seaplane base with an instrument approach to land in IMC don't you? A seaplane wouldn't be much use to me if I had to call in sick every time it got cloudy.

Well, I'll try to keep this brief. First, the only time anyone can legally land in hard IMC is on a CAT III ILS, which you couldn't make one if you wanted to into the water (no ILS protected area, prohibitive expense, testing logistics, etc.). Let's say, you make a Cat I, which will have mins of about 200' and 3/4 mile (no credit given for lighting system). This sounds good, but look at Class G minimums, and you get 1 mile and clear of clouds. Min altitude in Class G overwater? Right, none (91.119c). You are more constrained by distance to any person, vessel, vehicle, or structure. I will...

point out that you are exempted for takeoff and landing. So you knocked a 1/4 mile off your vis (which nobody is measuring anyways). Not worth an ILS.  CAUTION: Flying a seaplane around in any conditions even remotely close to those mentioned above is not a good idea for the inexperienced… and by inexperienced I mean anyone that doesn't do it for a living. You are more likely to get yourself killed than violated. Tack on the price of a seaplane and maintenance  (about 3-4x that of an equivalent landplane) and you are much better off (and probably safer) in the bay area just hiring a town car.

I can’t remember the last time I saw a small, IFR-certified aircraft that didn’t have the ability to do RNAV/LPV approaches. It’s the airliners that can’t do them.

Well there is at least one case where this is true and it's used by the military - A Case 3 instrument approach to an aircraft carrier.

The approach is essentially equivalent to a terrestrial military TACAN approach coupled with a precision microwave landing system on final approach.  See the basic plate below.  Both IMC and night VFR conditions require a Case 3 approach per NATOPS.

As to civilian seaplane bases, the answer is actually, yes, some do.  One example is the GPS RNAV approach into Greenville Seaport (52B) in Greenville, Maine.

As for an instrument approach to land a seaplane at a arbitrary point in the middle of the ocean, no, I am not aware of anything like that.  Maybe one day an advanced version of GPS or some similar satellite based navigation system coupled with an advanced INS could allow a pilot to arbitrarily pick a point then create a custom approach which provided lateral and vertical guidance to the landing site.  Even if you could do this, you still face unknown hazards like surface winds and waves (seaplanes were never made to brave rough open ocean swells).  It would be virtually impossible to safely land on a moonless night.

@ymb1 Well, imagine a pilot trying to land from the wrong end of the carrier...

it is not possible to land to sea in IFR condition. You need to see and you have to land your plane parallel to waves.

the other answer disproves you.

@Federico, I see the other answer totally agreeing! That is, as long as the “sea” is qualified to “open sea”. The seaplane bases with IFR approaches are sheltered ports, not open sea. On open sea, you do have to mind the waves.

@JanHudec but this answer does not make that distinction

@Federico, well, that depends on how the poster meant it; I wouldn't normally call a bay a sea.

@JanHudec but you had to specify "open sea" to make your point.

@Federico, because it was obvious that you did not understand "sea" as implying "open" while I did. It's ambiguous.

can u expand ur answer it doesnt bring anything new


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What is the "No Step" flap thingy used for on an A320 engine?
There is a small plate sticking out of an A320's engine narcelle with "No Step" written on it. I tried taking a picture of it, which in retrospect didn't turn out as good as I wanted it to be, but I hope it helps:

Anyway, I'm wondering what it's used for? It's only on one side towards the aircrafts body and it seems to be too small to be an aerodynamic surface. I suspect it's used during maintenance?

does this answer your question? http://aviation.stackexchange.com/q/11708/1467

@Federico It does indeed. I knew that must've been asked here before, I just had no idea what to enter besides "No step thingy". I also had no idea it was on other engines as well. I live in a small secluded world where all flights I take are on an A320 family plane.

It's a vortex generator explained in full in this question
"NO STEP" means don't put your foot on it. It's too weak to hold a person's weight and would get damaged.

Ah, that explains why I couldn't find anything when looking for this. Thanks :)

The person could also be damaged (though the mechanic who has to fix the bent metal doesn't really care about that - they're probably likely to inflict more damage on you for breaking their airplane).


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How does the Airbus flight computer's voting system work?
From what I've read about Airbus planes is that their fly-by-wire system works by having three different flight control computers calculate what the plane should be doing and then vote on all the three outcomes about which to take. I would hope that they all give the same result all of the time, but as I understand it this is used to make sure that decisions based on faulty data doesn't scatter the plane across the ground. How does this actually work? How does the computer vote which result is the correct one, is it just a majority vote? What happens when all three outcomes disagree?
Also, is this system only in place when operating in normal law? And are they actually different software implementations or is the only difference that they are fed input from different redundant sensors?

Could you add more details, in order to improve the chance that responders can give more accurate information which is not already in the Airbus materials. for example: where have you read about the three flight control computers? generally speaking, the simplest way to choose which calculation to use, is to use the "middle one". Airbus training materials cover these topics in quite good detail. (all three outcomes are never completely equal (they may have small differences but there's always some difference).

@GürkanÇetin I'm a complete layperson so an answer telling me where I can find the information in publicly accessible Airbus docs would be much appreciated as well. Basically I find this fascinating from a software developers standpoint and I would like to know more about how Airbus is aiming to guarantee that the plane doesn't turn into pieces. The way I understand it the redundant flight computers play a part in that.

Here's one of them: http://www.smartcockpit.com/docs/A320-Flight_Controls.pdf

and this is another one: http://www.skybrary.aero/bookshelf/books/2313.pdf

@GürkanÇetin Thanks, I didn't know these documents were publically available. I know what I'll be reading on my transatlantic flight next week :)

Happy readings :) The internet has no end.. With some more effort, you can find almost all design related data for Airbus, Boeing, MD, Embraer aircraft(s) flight control systems (architecture, basic functions, etc). They are there.

As far as I read in various documents about A320 and remember them correctly:
There are three systems handling different parts of the primary flight control:

ELAC (elevator & aileron computer) controls pitch with elevators+trim and roll with ailerons.
SEC (spoiler & elevator computer) controls roll with spoilers and if ELACs fail, pitch with elevators+trim.
FAC (flight augmentation computer) implements yaw damper. Its command is added to the rudder command. In A320 rudder pedals have direct mechanical (hydraulic) link.

The ELAC and FAC are composed of two identical units each, the SEC of three identical units (but only two of the units are used as backup for ELAC).
Each unit is composed of two dissimilar computer boards. One used i386 CPU, the other m86k CPU and each has independently developed software (to minimize risk of the same software bug in both).
One of the boards in each pair calculates the output and the other one verifies it. If the verification fails, that unit declares fault and disconnects. This is the primary way of detecting faults.
I found no reference to comparing outputs of the identical units. Except for spoiler control there are only two output and the check board is more reliable way of detecting faults anyway.
If the FAC fails, the aircraft won't maintain coordinated flight well and will tend to dutch roll, but pilots can compensate it with rudder input.
If the SEC fails, the ELAC can handle control alone. Just spoilers won't be available.
If the ELAC fails, the SEC can take over too. Turning with spoilers may cause some more drag, but not a big problem.
If both ELAC and SEC fail, the pitch trim wheels have direct mechanical (hydraulic) link and roll can be controlled by rudder by taking advantage of the yaw stability. I don't think it was ever needed in practice. Note, that is for A320 family; which controls are available in case of failure of all flight computers is different in each Airbus type.

Update: since this answer was written, the accident Smartlynx A320 training flights MYX-9001 at Tallinn on Feb 28th 2018 happened where ELAC pitch control failed due to fault in the stabilizer actuator and then both SECs failed due to the command and monitor boards responding differently to a short bounce, causing downgrade all the way to mechanical law. In retrospect it seems to have been rather unwise for the crew to continue the training with the intermittent actuator errors they were getting.

As for data input, there are three ADIRUs (air data and inertial reference unit) and each flight computer takes input from all three and compares them. It needs two matching (similar within some limit) values to consider it trustworthy. If either more than one unit fails or no two agree, the flight computers degrade to alternate law or direct law.
In alternate law, the system stops providing alpha protection (stall), overspeed protection and, depending on what failed, other flight envelope protection (there are two kinds, with different protections lost).
In direct law it reverts to directly mapping the stick position to control surface position similar to mechanical controls. But most faults only result in alternate law except if all inertial references were lost (that would be very difficult to handle as it would mean unreliable attitude indicators too, but I don't think that ever happened either; the inertial systems are very reliable).
Alternate or direct law can also be entered if some control surface (or its actuator) fails and somewhere in the computer failure sequence, but I don't remember the exact condition (FACs calculate the limits, so if FACs fail, it will certainly degrade).

awesome answer!

This is exactly what i was looking for. Thank you very much, awesome answer indeed! :)

@JustSid: I checked some documents and it says only SECs are three, ELACs are only two, and does not mention comparing their outputs. It relies on the checking board for detecting faults; it's the more reliable method anyway.

Deleted and upvoted, thanks for the clarification.

How exactly does that handle software failure if the units are the same? If, theoretically, the ELAC and SEC both have a software error, leading to wrong calculations on some input (let's say only in the i386). If this input comes in, wouldn't ALL ELAC and SEC units declare faulty and stop working?

@Josef: ELAC and SEC have different software.

Sure, but they can both fail. Or there is a bug in the CPU that crashes on certain values. CPUs have a lot of bugs too, nowadays.

@Josef: Everything can fail at once and you can't do anything about it. The point is to make the failures independent to reduce the risk. A CPU may crash on certain value, but since they have different software, they won't both be seeing the same value at the same time. And then of course the units are well tested; they are known not to have lots of bugs. And if the testing reduced probability of failure to say 10⁻⁶/hour (it is likely less), probability of both failing goes down to 10⁻¹²/hour and that is and that is once in several thousand years even with the number flying.


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Is it possible to do a Live-stream web cam on an airplane over the oceans?
Just wondering if this technology exists or has been attempted. For positioning purposes.

Sure, if the airline is willing to pay for expensive satellite bandwidth. But a webcam wouldn't be useful for position information, you could just transmit the aircraft's position using ADS-B, ACARS or other methods. This question has a lot of relevant discussion.

I do not think that a webcam pointed at the ocean would provide any useful information for positioning.

A web cam pointed at the sea could help locate a ship in distress. If the people onboard the plane missed something on the video, people 'online' could point out 'Hey we saw a white speck in the left corner of screen . or something like that.

I'm confused about what you're asking here, especially after your comment: can you edit your question to be more specific? If you're thinking about using cameras on airliners to crowdsource finding ships in distress then feel free to ask about that, but right now your question seems very vague.

@NormLDude: The uses you consider do not makes sense for an airliner. You cannot identify a location looking at water. Identifying a ship from 10 km requires a powerful zoom, a clear weather, and only a small strip of water can be scanned. Many aircraft already send their GPS position to ground stations (ADS-B), however ground stations are sparse and only on lands. The plan is to use satellites to complement (the passenger will indeed pay the additional cost).

@mins.  Not to mention that the percentage of the oceans surface area crossed by aircraft is tiny.

No, there is no mid-ocean real-time streaming video useful for positioning purposes.

There are multiple questions here
Just wondering if this technology exists: Yes it does, providers like GoGo Inflight Internet have systems in place to have internet connectivity on an airplane. You could, technically speaking use this channel to stream anything you wanted but you may hog bandwidth or see problems because of the way the bandwidth is allocated. From a purely technical standpoint it can be done.
For positioning purposes: This could be taken to mean a few things.

Where is the plane: In this case video does nothing for you. There are no points of reference over the ocean so capturing footage of nothing leaves you with nothing.
Whats beneath the plane: For the sake of SAR or simply checking up whats out there this can be done and as a matter of fact has been done. For what its worth the tech to do this is not cheap nor is it simple. Check out this question about the SR-71 which covers how the cameras worked pretty well. In short you need a camera that moves in sync with the plane so that the pictures come out clear. Granted the SR-71 used a film camera which could be replaced with a digital system the photographic element is not nearly as important as the system that moves it to make sure the photo is correct. Even then the SR-71 was sent on very specific missions over very specific places and the film was carefully post processed. Lets say you installed one of these systems on every plane out there (in one of those perfect worlds) the amount of time/computing power to process all that video in time to make use of it would be pushing if not over the limits of modern technology.

For reference here is the camera that the SR-71 used to photograph lots of area (they had another higher res one for smaller things). you could cut down the size these days since you would not need those huge film rolls for a digital system. Either way its a large precision machine (that Im sure is not cheap to operate or maintain).

...and very little ocean area is flown over by routine airline traffic.   For example, look at the North Atlantic Tracks.  The chances of an airliner spotting a ship, even if looking, are tiny.


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Could a plane land inside or on top of another plane in flight?
I'm sorry if this is a little fantastical, but I'm just wondering if this type of maneuver is possible in any way. I understand if this question gets pulled. I'm trying to make a little action-animation-cgi and just wondering if this was ever a last-resort option for maybe two planes, maybe one is a bit larger than the other but maybe similar speeds could be maintained for the one plane that is running out of fuel could latch-on somehow.

Interesting.. We do launch aircraft from other aircraft, but I think the closest we are to "landing" one is aerial refuling. Not that I have any clue whatsoever though. :)

I could have added blimps, or airships (Lockheed thing), but I think the speed might be bit more complicated. Don't waste your time trying to math this out, it is just an idea.

So there would have to be a specific 'holding' or flat area to land on the plane I guess. Planes landing on boats or buses, even seen a airliner do a barrel roll (Denzel movie) but seeing another plane land on another plane, that could be kinda cool imho

If you matched speeds you could bring the landing gear in contact with the other plane. The question then is what would keep you there? I wouldn't call it "landing" unless you could stop flying. If you take the planes Mike Sowsun posted in his answer you could theoretically do the same thing from the top. Once the smaller plane was in some way attached you could say it "landed."

"wondering if this was ever a last-resort option for maybe two planes, ... for the one plane that is running out of fuel could latch-on" - See in-flight refuelling instead.

I bet Michael Knight could do it.

Related: http://aviation.stackexchange.com/questions/9854/can-a-fighter-jet-land-on-a-modified-airliner

While flying a 150 with a student we overflew the Sanyo blimp preparing to moor at my local field.  I asked myself the same question.  Assuming that the top surface of the blimp could support an airplane (probably can't), I dont think the craft is long enough to make a successful landing.  Given that it's also a flying machine, turning into the wind like a carrier wouln't help as the "headwind" (or tailwind for that matter) would have zero net effect.

I could swear I've seen pictures of a Piper Cub landing on top of another plane in flight. They had some kind of brackets on top of the second plane to strap down the Cub's landing gear. Of course, now I can't find anything online about it.

Landing in another plane would probably be extremely tricky. The smaller plane would still need airflow over its wings to create lift during the landing, so either the bigger plane would need to be tube-like, or it would need to somehow blow air through the landing bay and at the small plane. Either way, you're basically talking about a flying wind tunnel.

You mean like in Executive Decision ?

The airforce used to have Parasite Fighters that could be launched and then recovered by a larger aircraft.
Parasite Fighter

The Space Shuttle was routinely transported on the back of two specially equipped B747s.
Shuttle Carrier Aircraft

Nice pic :)  Although the question is more of a technical question, like if a plane could support another smaller plane on the top part, or inside.

Yes, technically a large aircraft can support another aircraft on top or inside. The weight would not be a problem for a large enough aircraft.

Excellent examples, Mike, however @NormLDude asked specifically about landing on top of the mother ship. All of the examples at the "Parasite Aircraft" Wiki page were launched from and caught from below the mother ship (a couple launched from above, but were not air-recovered). And for the SCA, loading/unloading was a very static exercise done on the ground with cranes.

The Navy has already done this using airships.  Here is a video showing launch and recovery. USS Macon launching a recovering aircraft.

Let me take a stab at the "inside another plane part".
One of the smallest trainer planes is the Cessna 172, with a wingspan of 36'.
One of the biggest cargo planes is the C-5 Galaxy, with an inside diameter of 19'.
So right there, you'd be hard-pressed to find a small plane that could even fit inside a larger plane.
The next problem is airspeed:  The C-172 needs to travel at least 40 kts to maintain flight, and this is 40kts relative to the air it is in, so if it were inside the cargohold of a larger plane, it needs to be traveling 40 kts faster than the larger plane.  I couldn't find a canonical reference for the C5 Galaxy's minimum speed, but I can estimate it around 140 kts.
This means that on approach to the host-plane, the smaller plane would need to be doing about 180 kts relative to the outside air.  That is around the fastest speed the Cessna is capable of; it would need to be in a considerable descent at full power to achieve that speed. The propeller would be turning about 2700 RPM.
On the transition from outside the host to inside the host, the small plane needs to slow its propeller from 180kts (2700 RPM) to only 40 kts(1200 RPM).  The propeller needs to slow to landing speed nearly instantaneously, otherwise the fast propeller would accelerate the smaller plane relative to its host.
Essentially, the parasite-plane would need to be in a considerable descent at full power to make an approach speed.  Then as it enters the host-plane, it would need to level off and slow its propeller drastically. This seems incredibly risky.
Once you're inside the larger plane, the Cessna has a published landing roll out of about 500 feet.   The C-5 Galaxy has a length of around 250 feet. So you'd need some sort of arrester cable system, like found on aircraft carriers.
So, in summary:

Most big planes are not big enough to fit small planes inside
Small planes cannot go fast enough to catch up to a big plane's minimum speed.
The airspeed transition from outside a big plane to inside a big plane is extreme and sudden.
There isn't a lot of space in a big plane for a landing roll out.

So, I would generally say "No", landing inside a larger plane seems impossible.

You don't have to keep flying once you enter the door, and accelerating relative to the air in the plane would take time anyway, so there won't be much relative speed to arrest.  There's a version of the BD-5 with a 14'3" wingspan, while the Stits SA-2A biplane spanned just over 7 feet.  Yves Rossy would fit, though I'm not sure you could talk even him into flying into a net (grabbing a refueling probe while flying a wingsuit would be... interesting).  Wake turbulence could need some thought at least in planning the approach.

It seems likely that before anyone contemplates landing one plane inside another, both the mothership and the parasite would be specially modified or designed for that task. So the parasite could have a smaller wingspan and more power than a 172, for example. But the idea of landing inside another aircraft still seems like a bad idea--it would be much easier to modify the mothership to allow the parasite to attach outside first.

Once you start talking about specially modified / designed for a specific task, pretty much anything that was "impossible" becomes "possible".  But this site is about real aviation that actually exists.   Even if someone technically could make a parasite/mothership combo, I just don't see any scenario that makes it genuinely useful.

It's not that doing so would be obviously useful, or easy but that the specific arguments you raised are mostly erroneous.  Even the "airspeed transition" one is a poor match for the likely actual issue of turbulence behind the carrier aircraft and at its open door.

"at least 40 kts to maintain flight, and this is 40kts relative to the air it is in, so if it were inside the cargohold of a larger plane, it needs to be traveling 40 kts faster than the larger plane"  This conclusion is wrong because the question specifies that the flight should not be maintained.  The landing distance could be much shorter than normal.

"Small planes cannot go fast" This conflates plane size with engine type.  Small jet planes and rocket planes are possible.

Something similar has happened before; see Pardo's Push.
Two F-4 fighters were on a mission over Vietnam when they got hit with anti-aircraft fire. One jet lost too much fuel to be able to make it back to a tanker or air base. The pilot of the other jet, Captain Bob Pardo, used the canopy of his jet to push on the tail hook of the other jet.
This was certainly a "last resort" situation. It slowed their rate of descent just enough for them to fly further back to base. Both pilots bailed out and were rescued, eventually being awarded the Silver Star.

This was my grandpa landing on another plane in the Canadian airforce, I’m not sure how. He taught pilots in the airforce but passed when he was 40 before my dad was born so all I have to go on is what my grandma told me. My grandma would often show me this and tell me that he landed on top of another plane when I was a little girl.  I found this in an old photo album last week and googled to see how and if there were other pictures but only found this thread.

The two planes appear to have had different directional attitudes when this manoeuvre took place; I wonder how is that feasible (by producing a sideslip maybe?). Also, how did the propeller not get damaged? and lastly, why would any Air Force consent this? (even Red Bull would probably think twice!) It would be greatly appreciated if you could edit your answer to add more details if possible. Btw welcome to ASE!

I think a crane was used.


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If an aircraft were to fly ninety degrees upwards, at what altitude does weightlessness of space begin?
Imagining the pilots have fainted in the cockpit and are pointing the aircraft upwards at approximately 90° . Not sure if it would get really cold, or really hot before entering "space". Or if a normal aircraft, like a Boeing777 or Boeing787 could make it past a certain altitude without certain pressurization issues.

Comments are not for extended discussion; this conversation has been moved to chat.

The weightless experienced in earth orbit is a balance between centrifugal effect of rotation around the earth, and the gravitational force of the earth. Because that balance is seldom perfect, we call it microgravity.
Other ways microgravity can be obtained is by flying "parabolic flight paths" where the aircraft is accelerated towards the earth with similar acceleration as gravity, hence within the reference of the aircraft, the acceleration of the aircraft matches the acceleration of the earth.
Back to the height question, one would have to go well beyond the bulk of the atmosphere, and certainly not in a jet, which requires atmospheric oxygen to help power it. In a very general sense, for low earth orbit (LEO) to be reached, the speeds required are on the order of 17,000 mph.
So since the velocity of the object escaping from earth must reach a point where the object has a centrifugal effect equal to the attraction of the object by the gravitation of earth, higher velocities result in higher orbits.  Kepler applied the inverse square relationship of gravity between two bodies, resulting in what now is known as a combination of Newton's Laws and Kepler's laws. These were intended to deal with celestial bodies, and does not have provision for the high energy requirement of fighting a dense atmosphere. Once an object is substantially outside the atmosphere, then the numbers work. The Satellite Tool Kit (commonly called STK) is a useful tool for determining orbits, velocities and interrelationships between objects.
A 787 would not go fast enough to balance the gravitational forces of the earth, and because it could not escape the atmospheric drag of the atmosphere, it would have a very high power requirement to reach high speeds. So there is no point that microgravity would be achieved in sustained flight. Therefore, the conditions in this question cannot be met.

I'm sorry, but whilst your premise is right, your undestanding of the physics is just wrong. Centrifugal forces is categorically not a real thing and things in orbit are not weightless. In fact, on the ISS, gravity is still about 90% that on earth, and as such, their weight is near identical. What is actually happening in orbit is simply that due to your speed and the curvature of the earth the surface is falling away from you at the same speed as gravity pulls you down. There is an imbalanced resultant force here which is causing your constant accellaration (velocity is a vector quantity)

@JamesHughes While this answer does have some technicalities ill-defined, your comment is also not trivially true. To quote Wikipedia: "There is also a rival tradition [...] which sees weight as [...] a measure of the magnitude of the reaction force exerted on a body. Typically, in measuring an object's weight, the object is placed on scales at rest with respect to the earth, but the definition can be extended to other states of motion. Thus, in a state of free fall, the weight would be zero. In this second sense of weight, terrestrial objects can be weightless".

@JamesHughes this is not Physics SE, and I intentionally answered in the context of Aviation SE, and addressed the response to the poster.  Perhaps I used the old school centrifugal force term, but it is still taught today in high school.  Perhaps a more correct term would have been centrifugal effect which more accurately describes the inertial property of an object in orbit.

Centrifugal effect is measurable, in that a 200# person on the poles, will weigh 199 # on the equator due to the rotation around the earth with a velocity of about 1000mph (~435m/s).

There is such a thing as "centrifugal force" in non-inertial reference frames in classical mechanics. While centrifugal force is often called a "fictitious" force in that context, the term "centrifugal force" is still used; and for that matter, gravity itself is a "fictitious" force in general relativity: https://physics.stackexchange.com/questions/33875/gravitation-is-not-force -- Add the obligatory link to https://xkcd.com/123/ -- Having said all that, I think "effect" fits better than "force" did in the first sentence of this answer.

Ha ha, just think, if the earth spun something like 200 times faster, then one could jump up at the equator, and stay in orbit a few feet off the ground. And one could travel to the poles where they would "weigh" their full weight, without any benefit of less than orbital offset due to the inertial effects often called centrifugal force or centrifugal effect. Of course this experiment would not work well if the atmosphere didn't spin with the surface of the earth!

Let's assume that the thrust of the Boeing 777 is approximately 20% of the weight, and that with no lift in vertical flight the average drag during the zoom climb is 5% of the weight of the aircraft.
Let's say we start the 90 degree climb at a cruise speed of 250 m/s (900 km/h), and the deceleration due to gravity corrected for thrust and drag will be $ 85 \% \times 9.81 = 8.35$ m/s2.
After $\frac{V}{a} = \frac{250}{0.85 \cdot 9.81} \approx 30$ seconds the aircraft would have climbed approximately 3.75 km (12 300 feet) and lost all its speed. If we would start this zoom climb at 35 000 feet (a common cruise level for the B777), the apogee would be at approximately 47 000 feet (approx. 14.3 km) which is well below 100 km, the height of the arbitrary line between atmosphere and space.  Since it is above the maximum certified altitude of 43 100 ft, you may expect some pressurisation problems.
It turns out that the B777 is not suitable for space flight.
After having reached the apogee, the aircraft will start to fall back to earth.
If the thrust is cut before the fall starts, then weightlessness will be at the moment the speed is 0 (zero-drag). At that precise moment the acceleration will be only due to gravity since there are no external forces applied to the aircraft; for a moment it will be weightless.
The outside temperature is approximately the same as at 35 000 level and the pull of gravity is not changed significantly either.
A far more efficient way of achieving weightlessness in a B777 is using a parabolic flight path.

"It turns out that the B777 is not suitable for space flight" What a pity:(

Weightlessness doesn't come from height, it comes from lateral speed. ISS is not very far away, it just flies really fast.
The apparently humorous statement from Hithchiker's guide to Galaxy is not actually a joke at all when you replace "flying" with "orbiting": "There is an art to flying, or rather a knack. The knack lies in learning how to throw yourself at the ground and miss. ... Clearly, it is this second part, the missing, that presents the difficulties."
This is pretty much what satellite in orbit does: it flies so fast sideways that before it can hit the ground, it have already missed the Earth.
So, your Boeing 777 doesn't actually have to fly very high to achieve weightlessness. It "merely" has to fly very, very fast. For ground level, that's approximately 7910m/s, or a little over Mach 23. Sadly, even ballistic missiles can barely do half of that.
SpaceShipOne does exactly what you propose: it flies nearly vertically very high (over 100km), but even there it doesn't achieve weightlessness. As high as it seems, still 7850m/s is required to stay there (the difference in orbital speed isn't big, because 100km isn't much when you're adding it to Earth's 6371km radius). SSO's lateral speed is close to zero instead, so when the rocket motor burns out, it falls back to Earth like a rock. The people inside experience weightlessness for a while then, but it's no different than what you experience when jumping down.
XKCD link, as requested

The biggest flaw in your question is your understanding of weightlessness.
It is often misconstrued as a state when in fact it is actually the lack of a state.
Weightless simply means you can not feel gravity, it does not mean gravity is not there.
When you stand on the ground you feel heavy because gravity is pulling you towards the centre of the earth, but, obviously, the ground is not going to move out of the way, so pushes back with an equal force. The weight you feel is that compression between the two opposing forces and the drag of your bones and organs being held up by your body tissue.
If however you are falling in a vacuum, there is no opposing force, and you feel no weight. Similarly, if you are in an elevator, and the cable breaks, you will fall at the same rate as the elevator and will feel weightless. Or at least, you will for a short while.
If you jump off a cliff, ignoring air resistance, you will be weightless till you hit the ground. If you run really fast before you jump, you will fall and hit the ground further away from the base of the cliff. If the cliff is high enough and you run really, really, fast, you will fall slower than the curvature of the earth, and will never hit it. You will then be in orbit.
None of that has anything to do with reaching a specific height. (Well other than the crashing into the side of Mt Everest part in the last example.)

First of all, what do we mean by vertical?  If we mean 90 degrees straight up staying directly over the runway, you are still rotating with the Earth at a rate of one circle every 24 hours.  So the answer to your question in that case would be geosynchronous altitude.
This assumes that your plane is space-capable, has plenty of energetic fuel, and is taking off from the equator.  If you are not starting at 0 degrees latitude, celestial mechanics would tend to force you to enter a great circle route, crossing the equator twice with each "orbit".  This means part of your fuel would have to be spent maintaining your latitude; more so the higher you go.
If you don't mind "going off the vertical" by the time you get up there, you would again achieve weightlessness by the time your initial rotational speed (from the fact that the ground was moving to begin with) is fast enough to go into orbit.
However, if by vertical you mean exactly straight up while the Earth turns under you (sidereally vertical), or if you take off from either pole, then you never break free of Earth's gravity -- you could be as far away as Arcturus and there would still be an infinitesimal amount of gravity pulling you back to Earth.
But for practical purposes, you are "free" from Earth's gravity if the contribution from Earth's mass is trivial compared to other nearby bodies.

While this is a correct explanation of orbiting body, it doesn't really apply to the context assumed in the question. You need to take into consideration how a commercial airliner flies, with a maximum altitude of about 11 km, else this doesn't really answer the question.

I was assuming the question was about what would happen if the aircraft could fly indefinitely upward -- sort of like how much wood could a woodchuck chuck if a woodchuck could chuck wood.  If not, it wouldn't be much of a question.  Keep in mind an actual commercial aircraft cannot fly straight up -- with the exception that if you turn towards the zenith, you can proceed so for a short time, losing momentum swiftly.  In that context, my answer is the best one of all.

Also, the question was at what altitude would you achieve weightlessness -- not about what you would have to do to simulate a zero-g environment.  Too much XYing!

I stand corrected. Looking again, the question is indeed not related to aviation, except for pressurization. The geosynchronous orbit height is the answer. +1

@mins, geosync is not a special orbit with respect to microgravity effects. Rather it is merely the point that the orbital rotation matches the rotation of the earth.  If the earth changed mass, or spun at a different rate, then the altitude of a geosync orbit would change. Jennifer is correct in that the microgravity effect comes into play at the time that the inertial centrifugal effect matches the pull of planetary gravity, because at that point there is equilibrum. That equilibrium will exist for any object, regardless of mass (assuming non-planetary masses which changes things a little).

@mongo: As the OP assumes a vertical climb, GEO is the only possible orbit, else some positive/negative horizontal velocity must be provided.

@mins, the OP references altitude, not orbit. Furthermore, if one chose to place an object in geosync orbit then they would have to progress through a series (or continuum) of orbits with increasing radii until arriving at geosync. A vertical climb is not orbital insertion. To your last point, a horizontal velocity must be imparted to accomplish the geosync orbit.

@mongo: Please re-read the first paragraph of the above answer. If you have more comments, I suggest using the chat.

If by "vertical climb" we mean staying over the same point on the ground (as stated in my answer) then you already have the horizontal velocity (which you would have to add to because the higher you go, the more horizontal velocity you would need to stay over the runway).  If you then go straight up and up, you will eventually reach the point where you can just shut off the engines and be in orbit.  Of course, this is a terrific waste of fuel -- it's much easier to push your way into higher and higher orbits, gradually.  But it can be done this way.

Weightlessness as observed in the near space (ISS, etc) is not a result of the Earth being far enough, the Earth is still close enough and would pull with the gravity. The spacecraft is in weightlessness because of the path (trajectory) it follows (including speed changes also, not just co-ordinates).
Hence all you need to get the weightlessness is to fly a certain pre-computed path that an artillery shell would follow if launched with the same speed and direction as the aircraft is currently flying. Looks like Airbus 300 can do this. The company site also claims zero gravity. Unfortunately, the aircraft is too low to complete the whole orbit around the Earth because the planet is too big and gets into the way.

As seen from the diagram, the path might be within the capabilities of the aircraft.
(from Wikipedia Commons)

the "zero-g" situation nominally exists when there is either an opposing "force" to the "force" of gravity, OR, when there is no force against the "force" of gravity. Shooting parabolic paths, provides for a short term acceleration of the aircraft to match the acceleration of gravity, so the aircraft has no opposition to gravity. Orbit is different, in that there is a centrifugal effect (some call a force) which opposes gravity, causing a local balance of those two effects, and creating a place of zero-g. A plane in a dive will not approach the velocity needed for inertial balance.

Because a plane cannot fly straight up, it will stall and fall until the pilot can resolve the emergency.  During that time, all passengers and baggage will appear to be weightless because they are falling at the same speed as the plane.

"Because a plane cannot fly straight up" mh, depends which plane you talking about.

As explained in another answers, weightlessness in orbit doesn't appear because of high altitude. But answering the actual question "what if" an aircraft could climb vertically without limits - the answer is - few tens of thousands km or miles. The gravity of Earth extends to infinity and you can never truly escape it, but it decreases quickly beyond certain distance, so the passengers could feel weightless even without orbiting.


text:
How to maintain straight and level flight after a climb?
I'm new to flying and I have this question.
Suppose I'm ascending to 2000 ft. at 67 kts and 2450 RPM. When I reach 2000 ft. maybe the plane wants to keep going up, even after I lower the RPM TO 2200, for example.
How do I make the plane flight straight and level at 2000 ft? With the trim and keeping 2200 RPM, or lowering the RPM to, for example, 2100?

It all depends...
There will be some minimum power required to maintain level flight at any given altitude. If you're applying more power than that then you can trim for level flight at any power setting. More power means faster flight, higher fuel consumption and less range, while less power will save fuel and take longer. It's really your call, but I'm sure your flying school will opt for economy every time!
You may also find that some light aircraft have prohibited ranges for RPM. Some Robin aircraft, for example, prohibit continuous operation in a band just above 2000rpm (don't remember the exact details) so you'd have to trim at a power setting below or above that band.

More power means faster flight, higher fuel flow rate, but not necessarily less range. There is optimal speed which gives longest range and deviating from it in either direction reduces the range.

Attitude + Power = Performance
Performance is your airspeed, climb/descend rate, and rate of turn.
Attitude is pitch, bank, and yaw.
Power is RPM, if you have fixed pitch prop.
So how do you transition from climb to level flight? When you reach 2000ft, or maybe 1950, push forward on the yoke or control stick to about level pitch, check the altimeter to make sure you're at 2000ft, not climbing or descending.  Your airspeed will start going up. Maintain forward pressure, it is going to increase as the speed increases (you can put some trim in to help).  Once you reach the desired airspeed, reduce throttle to keep the airspeed constant.  Then use trim to take out the pressure on the yoke.
Now this will give you a workout, because if you don't know exact RPM that you need to maintain desired speed, you will be chasing it and working the yoke to keep constant altitude.  So I would lower the nose after the climb, let it accelerate, then lower RPM to 2200 and keep the plane in level flight with the yoke and trim.  After a few minutes when everything is stabilized, I would note the airspeed.  Then I would reduce RPM to 2100, retrim nose up to keep level, wait a few more minutes and note the airspeed for 2100 RPM.  And so forth.  You can build a table of RPM and airspeed.  Then, next time you fly, you will just set the correct RPM for desired airspeed.

I like this, but would say to adjust the trim as needed while accelerating, and not wait until you reach your desired airspeed to adjust it.

You need to reduce power or lower the nose.
Most likely you will want to travel faster than 67kts so you will need to lower the nose and allow the airplane to accelerate.  As it accelerates, you will need to trim the airplane.
Proper trim technique requires you set the pitch and power for phase of flight and hold that with yoke pressure.  You will then trim off that pressure with the trim wheel.

Leveling out and accelerating to cruise speed is something done after every take off and will become routine.  The best way to experience it is to take a lesson!
One way to make it easier is to "round off" to your desired altitude starting from 500 feet below. You are climbing at 67 knots, certainly do not want to yank back power with your nose in the air.  So you start working the elevator and pitch control (yoke or stick) together.  This will become much easier with time.
Start by easing yoke forward. Speed will increase.  Ease off throttle.  Reach altitude and hold with yoke. (I flew a 172 so it's yoke). Speed will increase.  Ease off throttle.  After practice you will hear and see good cruise rpms, at first keep working at it.
It is worse to miss altitude than to be off on speed by 5 knots.  Try to hold altitude, and work for desired speed with small throttle and pitch changes.
(Don't forget to continue scanning around you as well!)
Once at altitude and speed, set trim for cruise and lean mixture if needed. Enjoy your flight.


text:
How to perform climbs and descents?
I have known that when you climb, you push the throttle to maximum and adjust the vertical speed with the commands (elevator) and the trim. Then when you reach the desired altitude you pull the throttle to cruise RPM setting (2350 for example) and it will maintain level.
For descents you pull the throttle to decrease RPM and you adjust your vertical speed with the commands and the trim again. Then when you reach the desired altitude you push the throttle to cruise RPM setting again and it will maintain level.
Obviously, using the carb heat (hot when lowering down the RPM and cold when increasing).
Is this correct? Is there a better way to perform this maneuvers?
Thanks, and sorry for the noob question.

"better" in what regard? Something cannot be "better" or "worse" without some basis for comparison (and some target to optimize)....

To enter the climb, it's power, attitude, trim. That is, start by setting climb power, as you already know. Remember to compensate for adverse yaw with the rudder. Immediately raise the nose to the climb attitude. Your instructor will teach you to recognise what this looks like in your aircraft: e.g. there might be a part of the dash that lines up with the horizon. Your airspeed will drop away to the desired airspeed for the climb: usually the best-climb speed. Once you're stable and balanced, at the correct airspeed, use the elevator trim to remove the need for pressure on the stick (or yoke).
To level off from the climb, it's attitude, power, trim. Start by lowering the nose to a level attitude. You need to start doing this before you reach the desired height, as it takes a few seconds to level off. Once you're level, the airplane will speed up. When you reach cruise speed, reduce the power to cruise power, and compensate for adverse yaw. Again, once you're stable at the correct speed, trim away any constant pressure on the stick.

A gliding or powered descent is similar. Power, attitude, trim to enter the descent. First set the reduced or idle power, and wait for your airspeed to decay to the correct airspeed for your descent. When you reach this airspeed, lower the nose to the glide attitude. Once the attitude and speed are stable, trim the aircraft.
To level off from the descent, it's power, attitude, trim again. Anticipate reaching the height you want, just like when climbing. Raise the throttle to cruise power. Adding power will naturally cause a pitch-up tendency in most airplanes, so let the nose come up to the level attitude, and then you'll need forward pressure on the stick to keep it there. Once you're stable and level, trim away that forward pressure.

So for most of these attitude changes, the sequence of events is power, attitude, trim (PAT as a mnemonic). Leaving a climb is different: there it's APT, because lowering the nose first lets the speed increase to the cruising speed before you need to touch the throttle. Whenever you change power, you'll need to use rudder to compensate for adverse yaw.
Generally, you maintain the correct rate of climb or descent by keeping the correct airspeed, not by looking at the vertical speed indicator. Usually you just want the best rate of climb or descent, and you'll already know what airspeed achieves this, so it's easy to just check the airspeed indicator every so often and pitch up if you're too fast, or down if you're too slow. There are circumstances where you need to keep a particular rate of climb or descent, but that's not the norm so it's not what's taught first to student pilots.
The Pilots' Operating Handbook for the aircraft you're flying will tell you when carb heat is needed. In general, you should have it hot when you're at idle or low power (i.e. in the descent), and cold the rest of the time.

Out of curiosity, say you want to climb so you increase power but you then neglect raising the nose to the climb attitude what would happen? Obviously, your airspeed increases & I assume that ought to lead to increased lift as well. So would you still climb but much slower vertical speed & a much higher airspeed than you want? Is this characterization correct? i.e. the attitude controls how much of your increased power translates to airspeed versus climb rate?

@curious_cat if you provided no elevator input and the airplane was trimmed when you increase power the airplane will pitch up and climb at whatever airspeed it was trimmed for and whatever climb rate that yields.

My instructor simply told me to use the AH on the PFD, aiming for 20 degrees on a takeoff climb and 10 for altitude changes in flight. Instrument readouts are much more reliable than lining up cockpit elements with the horizon (as both will change based on the plane, location and altoitude), and it's sitting right there in front of you so there's no good reason not to use it. You just have to make sure you continue to scan outside the cockpit and don't become too focused on the instruments.

And why they say "don't fly with the trim"? What does this mean?

@KeithS That's a good point, but for this answer I wouldn't use the AH as my main example, simply because many training aircraft don't have one. And of course if you find out on your take-off climb that the AH isn't working, you'll need to judge everything by eye until you can land, so you need to learn both ways anyway.

@nick It's best not to ask follow-up questions in the comments, because it doesn't give a lot of room for an answer, and it's harder for people to find. Post a new question, or if you have lots of small questions, maybe join us in [chat] and ask there.

ok thanks for the answers!


text:
What should you do when the pattern is crowded?
If you're approaching a non-towered field and you find that the pattern is too crowded to enter what do you do?
Provided you have adequate fuel to wait for an opening is there a standard place and altitude to hold?
What if you don't really have enough fuel to hold? Do you call on ctaf and say that you need to enter the pattern right away?

There are a couple of options for you.  Carlos' answer above is absolutely correct.  The FARs are helpful, as is AC 90-66A.  You should take a look at both of those.  AC 90-66A is informative, but not regulatory, and Part 91.113, 91.126 are regulatory but not particularly informative. 91.103 says you have to know everything about everything, which is true, but again, not helpful.
First the rules: Be safe, be alert and then proceed.  FAR 91.113(c) governs your low fuel question.  If you are below minimum fuel, and are in distress you have the right of way.  There is a difference between a fuel emergency, which is "I'm going to run out of gas unless I land now" and a minimum fuel advisory/situation which means, I got just enough gas to get where I'm going and get myself safely on the ground. If your fuel status is in doubt, it's an emergency.  Better to fill out that paperwork than the NTSB paperwork.
The other pertinent part of 91.113 is Aircraft, while on final approach to land or while landing have the right-of-way over other aircraft in flight or operating on the surface, except that they shall not take advantage of this rule to force an aircraft off the runway surface...
So, this means that the airplane on the final segment does have the right of way, over everybody but the emergency, but can't abuse the privilege.  So, if someone wants to disrupt a busy pattern with a long straight in final, it's legal, and traffic in the circuit do have to give way and turn the base behind him.
Next, FAR 91.126(b)(1) which says all turns shall be left hand in the traffic pattern, so plan left traffic unless the AFD (US) advises the traffic pattern is right hand turns.
AC 90-66A tells us to play nice in the pattern.  "It says Landing and takeoff should be accomplished on the operating runway most nearly aligned into the wind.  However, if a secondary runway is used, pilots using the secondary runway should avoid the flow of traffic to the runway most nearly aligned into the wind." (AC90-66A 8. Recommended Standard Traffic Pattern, (f)).  8 a. of the AC states, "Prior to entering the traffic pattern at an airport without an operating control tower, aircraft should avoid the flow of traffic until established on the entry leg."
I have seen pilots arrive at an airport with a busy stream of departures call and declare they were landing on the opposite direction of the departure runway.  They only have the right of way, if they are on final, but it doesn't strike me as a a nice thing (or safe thing) to do.
In a busy pattern, it is generally best to follow the AIM recommendations in the US, which is, unless otherwise noted, cross overhead the field and plan a 45 degree left downwind entry to join the traffic flow, watching traffic for spacing.  If in doubt, break off and try again.  Announce your intentions and position on the CTAF at all times, but be vigilant, as there may be some airplanes that do not have radios.  Which also means they will not be seen on TCAS/ADS-B or whatever.  It's probably a good idea to get an ads-b in receiver if you don't have one.  There's a homebrew one based on a raspberry pi that can be put together cheaply and mostly works.  Don't assume it will see all airplanes at all times.
Finally, once in the pattern go with the flow.  If the pattern is too full for your comfort, the general rule is to remain at least 500 ft above the pattern altitude and move away from the flow of traffic, but there may be others doing the same, so watch and listen.  Depending on the situation, you might want to divert to an alternate, nearby airport, buy some gas aand hope the pattern traffic is more to your liking when you come back.  Turn your base only after the next airplane in line has crossed abeam you and adjust your pattern accordingly.
You might want to practice with an instructor who has some experience in high density traffic areas before you go on your own.

I'm skeptical that "on final" automatically equals has right of way.  If a plane is left base for a runway (say 31) 500' above TDZE, and someone else calls 18 mile final (on a 3 degree, which would put them 5000' TDZE), does the straight in traffic really have the right of way? BTW, I picked 18 mile final, because apparently, that is the service volume of an ILS.

Say an IFR pilot wants to shoot an ILS 13 approach on a VFR day with 14kts of wind favoring runway 31. There are 4 planes doing left traffic for 31, 2 are on final (2nd in line extended downwind to fit a straight in for 31 fit in).  At some point (1000 AGL?) the IFR pilot should probably turn it into a "circle to land" by side stepping right a bit to join the left downwind for 31. It seems like some IFR pilots kind of forget if its not IMC, they still have a responsibility to "see and avoid" (and coordinate), and it's not going to be "taken care of" for them by ATC at an untowered airport.

As PIC if you don't feel comfortable entering congested airspace at a beehive airport, dont enter it.  It's that simple.  Divert to another airfield and attempt to return to the original airfield when the congestion abates.
If you anticipate high density traffic at an untowered airport, you should let other pilots know your intentions on the CTAF well in advance, generally at least 10NM from the field.  Closely monitor other CTAF pilot reports and start building a mental picture of traffic reporting their positions in the pattern, entering the pattern, or leaving it and begin to adjust your entry speed and position for appropriate separation and traffic flow.  Most other pilots will work with you on this and make changes to their traffic pattern to accommodate you as well.
If you have a fuel emergency or any other emergency for that matter and no other alternative is available, declare an emergency on the CTAF, indicate your position, altitude and intentions.  All other traffic in the pattern, by federal law, MUST give way for you

How would you find out when the congestion abates?

Monitor the CTAF at the field.

Emergencies and extreme traffic aside, orbit a mile or two out, state your intentions, and if you can't get in, ask if someone will let you in.  Without asking, it is common to do a crosswind 360 or extend the upwind leg to allow traffic to get in.
"Cessna 34A is extending upwind to allow Warrior circling south of Hometown to enter downwind. I have you in sight and will follow you in the pattern."
If you are a student on a solo, say so.  If you are unfamiliar with the field say so.  At a busy field there are usually several people who are familiar with the field or who are instructors, and will work to get you in.
"Cessna 34A is extending upwind to allow Warrior circling douth of Hometown to enter downwind.  Warrior, that will put you number three following a Maule and Citabria. That Citabria is now just turning base. Do you have him in sight?"
Practicing pattern courtesy is pretty easy, and as an instructor, you help show students how to negotiate from both sides.

BTW, sometimes a landmark is better than "two miles out"...like circling over the orange water tower.


text:
Can one become a CFI with a sport license?
Under FAA can a sport pilot get a cfi endorsement for LSA category only? If so do they need a medical?
Same questions for glider pilots.

Yes.
FAR 61.403.

To be eligible for a flight instructor certificate with a sport pilot rating you must:

(a) Be at least 18 years old.
(b) Be able to read, speak, write, and understand English. If you cannot read, speak, write, and understand English because of medical reasons, the FAA may place limits on your certificate as are necessary for the safe operation of light-sport aircraft.
(c) Hold at least a sport pilot certificate with category and class ratings or privileges, as applicable, that are appropriate to the flight instructor privileges sought.

A CFI never needs a medical certificate.  It is not required for the operation of that certificate.
A medical certificate may be required if the CFI has additional roles.  For example, the CFI also is acting as the legal pilot in command.  A sport pilot acting as the legal pilot in command does not need a medical.

FAR 61.23(b)

(b) Operations not requiring a medical certificate. A person is not required to hold a medical certificate—

(1) When exercising the privileges of a student pilot certificate while seeking—

(i) A sport pilot certificate with glider or balloon privileges; or
(ii) A pilot certificate with a glider category rating or balloon class rating;

(2) When exercising the privileges of a sport pilot certificate with privileges in a glider or balloon;
(3) When exercising the privileges of a pilot certificate with a glider category rating or balloon class rating in a glider or a balloon, as appropriate;
(4) When exercising the privileges of a flight instructor certificate with—

(i) A sport pilot rating in a glider or balloon; or
(ii) A glider category rating;

(5) When exercising the privileges of a flight instructor certificate if the person is not acting as  pilot in command or serving as a required pilot flight crewmember;
(6) When exercising the privileges of a ground instructor certificate;

To answer your questions.  A sport pilot or CFI will never need a medical because the operation does not require a medical but a valid driver's license.
A glider pilot or CFI never needs a medical or drivers license, if I am reading the regulation correctly.

A sport CFI does not require a CPL as does a regular CFI. The CPL requirement is found under part 61 subpart H

Subpart H—Flight Instructors Other than Flight Instructors With a Sport Pilot Rating

§61.183   Eligibility requirements.
    To be eligible for a flight instructor certificate or rating a person must:

(c) Hold either a commercial pilot certificate or airline transport pilot certificate with:

But subpart H specifies Other than Flight Instructors With a Sport Pilot Rating. The rules for sport CFI are found in subpart K where no such requirement is found. (see 61.403 above)

It does not relieve the pilot or CFI from adhering to FAR §61.53 Prohibition on operations during medical deficiency though.

Edited in a section to respond to the covers raised by Carlo Filicione.

Interesting.  However you would not be able to do flight instruction with a sport pilot in training for monetary compensation.  That would require a commercial pilot certificate and a Class II or above medical.

CFI's aren't being paid to fly an airplane but to instruct so a commercial certificate with medical is not required.

NO WAY.  If you are instructing, you are PIC until the guy in the left seat is a licensed pilot in that category and class of aircraft or type rated for the particular aircraft you are receiving instruction in.  See 14 CFR 61.315 and 91.3.  So this kind of presents a conundrum.  You can instruct other sport pilots but cannot do so for monetary compensation.

Actually in a sense it does, as it adds some clarity for sport pilot flight instruction in powered aircraft.

You should enhance your answer by highlighting what part of your post answer the question and by adding references for further readings.

Can you please cite the particular FAR's that apply to your answer? The FAA's notoriously ambiguous language is really throwing me here and I don't see anywhere they've published an AC or a legal interpretation on sport category.

Ok, I've figured this out. The rules for a CFI under part 61 subpart H require a CPL. But that subpart specifies in its title that it is for CFI's except CFI with a sport rating. The sport CFI rules are seperate under subpart K. In subpart K there is no CPL requirement per 61.403(c) as there is in 61.183(c) for regular CFI's


text:
Why is the layout of engine data different for CFM vs IAE engines on Airbuses?
I do a fair bit of simulated flying in consumer flight simulators (and have no real-world flying experience or formal education). Recently, I started putting hours into a supposedly study-level A319 simulation with the option to use either CFM56-586 engines or IAE 2524-A5 engines, and besides some very subtle performance differences and visual changes to the exterior model, I noticed that the layout of the virtual gauges on the upper ECAM display is different between the two models:

With CFM, the four data rows are (top to bottom) N1, EGT, N2, and FF, with no mention of EPR at all -- even if you select the ENG page on the lower ECAM.
With IAE, top to bottom, EPR is prioritized, followed by EGT and N1, with FF and N2 off to either side.

This seems consistent across some other Airbus addons in my simulator, including an A320 which ships only with CFM engines, as well as an A330 which ships with only RR engines, but retains the IAE layout.
What motivates this layout difference, and why is EPR prioritized above other engine data on IAE (and apparently RR) engine configurations?


text:
Are there any situations where it's beneficial to disable altitude reporting in TCAS?
Having altitude reporting enabled persistently seems like a pretty common-sense thing to do -- it enables RA in addition to TA, the benefits of which are fairly obvious. That said, most of the common transponders installed in GA and commercial aircraft have either the option to explicitly turn on (typically on GA) or to explicitly turn off (typically on commercial) altitude reporting.
Are there any normal or abnormal flight conditions which would make it impractical, unsafe, or impossible to have altitude reporting turned off?

Most GA aircraft don't have TCAS... Also since ADS-B was mandated, altitude reporting should be on all the time, you make an effort to turn it off...

Before ADS-B was mandated it was normal to disable transponder altitude reporting (and in some locations all reporting)  when on the ground as it saturated nearby TCAS. This was especially true with Mode A/C (ATCRBS) transponders. With ADS-B and many GA moving to Mode S (to get ADS-B) this is less of an issue as Mode S/ADS-B reports 'on ground'. Regardless, when airborne ATC will tell you to turn off altitude reporting if it is not accurate. If it's not accurate, it is a hazard as TCAS processes the data it gets - right or wrong.

The only time it would make sense as a normal or abnormal procedure to turn off altitude reporting is when your static encoder system is reporting no or incorrect altitude information. Especially in today’s world of mandatory Mode C and/or ADS-b in certain airspaces. ATC in those airspaces may have you cycle your system temporarily. Or, they could have you permanently  turn it off while you either land as soon as practicable or vacate the airspace until the system is repaired.


text:
Why is the ILS 36R approach at ZBAA over twice as long as a standard ILS approach?
I fly exclusively in a consumer flight simulator, so excuse me if there's an obvious answer to this.
ILS approach plates typically show a long, arrow-like symbol to indicate the part of an approach path where the localizer and glideslope signals can be captured by an approaching aircraft; most often, when there are no natural or manmade obstacles, for 3º glideslopes, this is about 9nm out from the threshold and 3000ft above the touchdown zone.
Looking at a chart for ZBAA's ILS 36R approach (which I'm not sure I can include in this post since this would likely violate Navigraph and/or Jeppesen's ToS), 36R specifically has this arrow extend out twice as far as either 01 or 36L — to about 25.5nm and 6890ft above the deck.
Why is this, and how common are such extended ILS approaches elsewhere in the world?

The other two ILS approaches for the parallel runways (36L and 01) also show the same long arrow on their respective charts. They all show the arrows of the other ILS approaches much shorter on their charts (I guess to avoid overlap).


text:
Why don't FMCs automatically include up-to-date oceanic route definitions?
In modern FMCs, flying an oceanic route requires inputting coordinates directly, often in obtuse formats (which are arguably prone to typos, but following proper procedure and a step through the route map generally prevent this being an issue).
Working with an example: North Atlantic Tracks, or NATs, are a set of roughly parallel airways in the North Atlantic, generally spanning from Canada's maritime provinces in the West to the Irish coast in the East. Because of frequently changing conditions in the North Atlantic, these routes undergo changes every few days, which are conveyed to pilots before each transatlantic flight.
Having said all that: modern FMCs are pretty clever, and they can (and regularly do) pull down data from the network, including in-flight weather data, for example. Why don't modern flight computers use this to also download oceanic route definitions if manual input is required rather than using a company route?